High power electric cables in underground installations are normally laid into protective conduits. In some instances, ground water may seep within the protective conduit which acts as a channel allowing the water to infiltrate underground enclosures containing sensitive electric equipment to which the protective conduit leads. To prevent such infiltration, a fluid-tight seal is formed at terminal points of the protective conduit, for example, where the conduit enters the underground enclosure. The fluid-tight seal is achieved by inserting in the end portion of the conduit a resilient plug of elastomeric material provided with bores through which the cables laid in the conduit can pass. By compressing the resilient plug, the latter is caused to expand and sealingly engage the inner surface of the conduit and the insulating sheaths of the cables, thereby forming the fluid-tight seal.
This seal is particularly effective with smooth surfaced cables having a generally circular cross sectional shape. However, in some type of installations, electric cables having an outer relief surface are being employed which cannot be sealed by the method used with ordinary smooth surfaced cables. The relief surface of the cable is formed by an array of electrical conductors, forming the neutral connection in a three-phase system for example, concentrically laid in a spaced apart relationship on the insulating sheath of the cable which surrounds the central core conductor. The traditional sealing methods are not suitable to this cable structure mainly because of the inability of the resilient plug to closely conform to the relief surface of the cable even when subjected to high compressive loads.
Attempts to solve this problem by injecting between the resilient plug and the relief surface of the cable paste-like sealing compounds, such as silicone or epoxy based products, have not met with success because the sealing compound has a tendency to harden and crack and is therefore unable to maintain a non-leaking union over long time periods.